Consider acetaldehyde, C2H4O. 2n + 2 = 6. (6-4)/2 = 1. Therefore, I would say that it has one degree of unsaturation, and the carbonyl group is the only candidate for an unsaturated group. However, when speaking of saturated versus unsaturated fatty acids, one always ignores the carboxylic acid, and instead one focuses on the hydrocarbon chain. Therefore, I think that there is some ambiguity in the way the term is used.